Intermodulation distortion is a type of signal distortion produced in nonlinear systems by amplitude modulation between signals of different frequencies. For example, intermodulation distortion is commonly produced in duplexers by amplitude modulation between transmitted and received signals, or between transmitted or received signals and external jammer signals.
Intermodulation distortion typically comprises a collection of distortion components at specific frequencies determined by the frequencies of the intermodulated signals (hereafter, “intermodulated signal frequencies”). These distortion components are referred to as intermodulation products. Intermodulation products are generally located both at harmonic frequencies of the intermodulated signal frequencies and at multiples of sum and difference frequencies of the intermodulated signal frequencies. For example, in a system having intermodulated signals, e.g., transmitted, received, and/or jammer signals, at frequencies F0, F1, F2, . . . , intermodulation products may be generated at frequencies nF0+mF1+pF2+ . . . , where n, m, and p are integers.
Intermodulation products can cause significant problems in many systems. As one example, in communication systems, some intermodulation products may fall within a frequency band of a transmitter or receiver, which can interfere with transmission or reception of a desired signal. As another example, intermodulation products may fall in sideband regions of a transmitted or received signal, which can increase the occupied bandwidth and lead to adjacent channel interference.
To address the above problems, some systems use filtering to eliminate or reduce the magnitude of certain intermodulation products. For example a notch filter may be placed at an antenna port of a duplexer to pass transmit and receive signals while notching out any jammer signals that may otherwise lead to intermodulation products. However, if the jammer signals are located close in frequency to the transmit and receive signals, the required notch filter will typically have a high insertion loss and may also give rise to its own third order intermodulation products. Additionally, the notch filter may need to be tunable to be able to reject the undesired jammer while passing the desired transmit and receive signals. Such a notch filter would need to be simultaneously tunable, linear, and highly frequency selective.
An alternative approach that does not require filtering is to use phase shifting to produce versions of intermodulation products that cancel each other by destructive interference. Examples of this approach are disclosed in U.S. patent application Ser. No. 13/341,797 entitled “Apparatus Having Double Phase Matched Configuration for Reducing Magnitude of Intermodulation Products”, the subject matter of which is hereby incorporated by reference. Although the disclosed examples can effectively reduce intermodulation products caused by the presence of jammer signals, they may require Wilkinson devices to split or recombine the applied transmit or jammer signals to absorb differential mode as well as common mode reflections that are caused by small mismatches at the ports where the signals are connected to the ports of the duplexer or filter. These differential mode reflections, if not absorbed by the Wilkinson combiner would otherwise be converted into frequency dependent common mode signals that would interfere with both the proper linear operation of the net duplexer or filter function of the overall topology as well as with the proper third-order intermodulation distortion phase cancellation scheme. The presence of the Wilkinson devices, however, can result in undesirable levels of insertion loss. For example, a Wilkinson device at a transmit port may result in undesired insertion loss in a signal path between the transmit port and an antenna port.
In view of the above and other shortcomings of conventional technologies, there is a general need for approaches that reduce, remove, or eliminate intermodulation products without creating undesirable amounts of insertion loss.